FIG. 1 illustrates a typical image scanning apparatus that provides a virtual microscope, which operates according to known principles.
The image scanning apparatus includes an imaging lens 1 which focuses light originating from a sample located on a slide 6 onto a line scan detector 2. The sample located on the slide 6 may be a biological specimen such as a tissue sample, for example.
As the detector 2 is a line scan detector, the image area is an elongate region 7 extending in a swathe width direction 5 (x-axis). The imaging lens 1 and the line scan detector 2 together make up an imaging system of the image scanning apparatus. The image scanning apparatus typically includes a slide mounting device configured to mount the slide 6 in the image scanning apparatus.
In order to produce an extended image over a large area of the sample located on the slide 6, the slide 6 is moved (by moving the slide mounting device) relative to the imaging lens 1 and line scan detector 2 in a scan length direction 8 (y-axis). In this sense the sample on the slide is “scanned” by the line scan detector 2.
In more detail, the image scanning apparatus is configured to image a surface of the sample located on the slide 6 in a plurality of swathes, wherein each swathe is formed by a group of scan lines, each scan line being acquired using the scan line detector 2 from a respective elongate region 7 of the surface of the sample extending in a scan width direction 5, wherein each group of scan lines is acquired whilst the slide 6 is moved relative to the scan line detector 2 in a scan length direction 8.
A focus setting of the image scanning apparatus may be adjusted, for example, by moving the imaging lens 1 along an imaging axis 9.
In a typical image scanning apparatus, an individual swathe acquired from the surface of the sample may be approximately 1 mm wide in the swathe width direction 5 and between 2 mm and 60 mm long in the swathe length direction 8. Multiple swathes can be combined to generate an image wider than the (approximately 1 mm) width of an individual swathe.
The present inventors observe that over the scale of 1 mm, the height variation of a typical biological sample (z-axis) does not typically exceed the depth of focus of the image scanning apparatus (typically in the region of 1 μm).
Further, the present inventors observe that during the acquisition of a swathe, a focus setting of the image scanning apparatus can be dynamically adjusted to maintain the sample in focus along the length of the sample in the scan length direction 8 (y-axis). Techniques for measuring and dynamically adjusting focus to maintain the sample in focus along the length of the sample in the scan length direction 8 are described in the literature, see, e.g., U.S. Pat. No. 7,485,834, WO2013/017855 and US2014/0071438.
Further, the present inventors have observed that it is not unknown for the height of a biological sample (z-axis) to vary more rapidly than the typical amount of 1 μm per mm discussed above. The present inventors have also observed that mechanical tolerances in typical image scanning apparatuses mean that the surface of the biological sample might be tilted (not parallel) in the scan width direction 5 relative to an imaging plane of the image scanning apparatus. For the scan length direction 8, this is not a problem because as described above the focus of the image scanning apparatus can be dynamically adjusted during the acquisition of a swathe. However, for the scan width direction 5, the possibility that the height of the biological sample might change more rapidly than the depth of focus of the image scanning apparatus across the width of a swathe is a problem, as it is not possible to dynamically adjust the focus so that the swathe is in focus across its width at one time.
The present disclosure has been devised in light of the above considerations.